Some will argue that it's the very first year of life. It's hazing that brings new parents to their knees: shock therapy for the 18 years ahead.
That's when parenting is most demanding, some say -- in those around-the-clock blurry days and nights.
But there's a reason the next year is called the "Terrible Twos" -- a time of meltdowns, the oft-repeated "No!" and high-speed chases, always potentially damaging to body and property.
If you escape the horribleness of the Twos, the Threes will get you, someone always chimes in.
So, when does it get easier? And which is the most difficult age to raise a child, truly?
As the mother of a newly minted 12-year-old girl, I keep hearing the words of an older colleague haunting my difficult days: Little people, little problems. Big people, big problems, he liked to say.
He may have been right.
The longest year may vary by gender and temperament. But a fair number of parents of daughters have warned me that there's a special kind of torment that happens around the ages of 12 to 14.
It was the sage Nora Ephron who advised getting a dog when your children hit their teens.
"When your children are teenagers, it's important to have a dog so that someone in the house is happy to see you," she wrote.
St. Louis-area clinical social worker Debbie Granick has two daughters, ages 12 and 14, and a 16-year-old son. If anyone is walking through the fire, she is.
"The worst year is the one you are least prepared for, when their behavior is most not what you had in mind," Granick said. "It's when you are put in the position of having to be (the) person you least like."
That person might be when you are sleep-deprived and angry at a colicky infant. Or it might be when you resist the impulse to slap a 12-year-old talking back to you, she said.
"It's when they hit on your vulnerabilities," Granick said. "Those years are difficult for everybody." It's typically when parents most often turn into a version of themselves they'd like to keep hidden from the world.
If you look at it through the lens of child development, however, there are biological and social reasons why certain years have a reputation for trying the patience of parents.
In the early years, when toddlers make the discovery that they are different and distinct people from you, they want to maximize that by testing every limit, Granick said. The period from about age 5 to 10 could be considered the golden age of childhood, she added, with a certain level of smoothness in routines.
But in early adolescence, a combination of hormonal changes, a more intense awareness of peers and less security in oneself throws that period into an abyss of frustration and power struggles.
You can't hold it against a baby when he cries all day. As much as it wears down your psyche and emotional reserves, intellectually you know it's not the child's fault. Even with defiant, crazy toddlers, you realize that they are just learning socially appropriate behavior.
But by those later tween and teen years, you begin to hope the previous decade of parenting has made some sort of impression. This may be why these stormy years can feel like the most dispiriting epoch of parenting: You start to question where you went wrong to end up with a child who barely tolerates your presence.
"You went right!" Granick says. "It is their developmental role to challenge everything you've taught them. You have raised a developmentally appropriate child, and you should look at some of the ways in which they are difficult as signs of success."
Even if it's considered "developmentally appropriate," it hurts when someone you love speaks to you disrespectfully and willfully ignores what you've asked them to do. When a child's behavior takes swings of intolerableness, it affects the parent as well. When a child begins to withdraw or shut you out, you wonder how long it may last.
Most of us can likely remember feeling terribly aggrieved by our parents during some phase of our childhood. And every parent of an adult child I've spoken to says that, eventually, by the grace of that relentless passage of time, even teenagers come back around.
Isn't there a way to bypass the worst of it? What can you do to shorten the later-stage Terribles so they don't last two, three, four or more years?
Granick suggests bearing in mind that when children are least lovable, it's precisely when they need the most love. Don't feel compelled to explain or respond to the content of their arguments, she said. But respond to the feeling they are trying to express.
"You don't have to correct the feeling," she said. You can revisit an episode later, outside the heat of the moment, to try to discuss a problem from another perspective. But, "you don't have to correct everything," she said.
It was the same approach we used when they hit the twos and threes: Pick your battles.
Would you swap a defiant, crazy toddler for a defiant, hormonal teenager?
Chances are, the toddler is cuddlier. But then again, the teenager is potty-trained.